Injection molding is the most important process for producing moldings from thermoplastics. This significance is due to the ability of injection molding to manufacture complex molding geometries in a single stage with high levels of reproducibility. Plastics finishing is largely unnecessary and a high degree of automation is possible. All manner of consumer goods and commodity articles are manufactured using injection molding of polyethylene thermoplastics.
To injection mold a part, polyethylene thermoplastic pellets, granules or powders are melted and injected under pressure into the cavity of a mold where the melted resin is solidified by cooling for subsequent removal. More detailed discussion of injection molding may be found in Ullman's Encyclopedia of Industrial Chemistry, vol. A20, Plastics Processing, pages 688–696 (VCH Publishers, 1992).
Blends of polyethylene resins have been proposed to improve physical properties, including impact strength, environmental stress crack resistance (ESCR), and chemical resistance.
U.S. Pat. No. 4,438,238 describes blends for extrusion processing, injection molding and films, where a combination of two ethylene-α-olefin copolymers with different densities, intrinsic viscosities and number of short chain branching per 1000 carbon atoms is attributed with such physical properties.
U.S. Pat. No. 4,461,873 describes ethylene polymer blends of a high molecular weight ethylene polymer, preferably a copolymer, and a low molecular weight ethylene polymer, preferably an ethylene homopolymer, for improved film properties and ESCR, useful in the manufacture of film, in blow molding techniques, or in the production of pipes and wire coating.
EP 0 423 962 describes ethylene polymer compositions particularly suitable for gas pipes, said to have improved ESCR, comprising two or more kinds of ethylene polymers different in average molecular weight, at least one of which is a high molecular weight ethylene polymer having an intrinsic viscosity of 4.5 to 10.0 dl/g in decalin at 135° C. and a density of 0.910 to 0.930 g/cm3, and another of which is a low molecular weight ethylene polymer having an intrinsic viscosity of 0.5 to 2.0 dl/g, as determined for the first polymer, and a density of 0.938 to 0.970 g/cm3.
U.S. Pat. No. 5,082,902 describes blends of linear polyethylenes for injection and rotational molding said to have reduced crystallization times with improved impact strength and ESCR. The blends comprise: (a) a first polymer having a density of from 0.85 to 0.95 g/cm3 and a melt index (MI) of 1 to 200 g/10 min; and (b) a second polymer having a density of 0.015 to 0.15 g/cm3 greater that the density of the first polymer and an MI differing by no more that 50% from the MI of the first polymer.
U.S. Pat. No. 5,306,775 describes polyethylene blends said to have a balance of properties for processing by any of the known thermoplastic processes, specifically including improved ESCR. These compositions have: (a) low molecular weight ethylene resins made using a chromium oxide-based catalyst and having a density at least 0.955 g/cm3 and MI) between 25 and 400 g/10 min; and (b) high molecular weight ethylene copolymer resins with a density not higher than 0.955 g/cm3 and a high load melt index (HLMI) between 0.1 and 50 g/10 min.
U.S. Pat. No. 5,382,631 describes linear interpolymer polyethylene blends having molecular weight distribution (Mw/Mn)≦3 and composition distribution (CDBI)≦50%, where the blends are generally free of fractions having higher molecular weight and lower average comonomer contents than other blend components. Improved properties for films, fibers, coatings, and molded articles are attributed to these blends. In one example, a first component is an ethylene-butene copolymer with a density of 0.9042 g/cm3, Mw/Mn of 2.3, and an MI of 4.0 dg/min and a second component is a high density polyethylene (HDPE) with a density of 0.9552 g/cm3, Mw/Mn of 2.8, and an MI of 5.0 dg/min. The blend is ascribed with improved tear strength characteristics.
There is a continuing need for polyethylene-based compositions having improved environmental stress cracking resistance, particularly those suitable for injection molding applications.